This application is based on Application No. 2002-000462, field in Japan on Jan. 7, 2002, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to a transformer for a gas insulated electric apparatus and, in particular, to a transformer for a gas insulated electric apparatus for a gas insulated switching device used in the power system transformation field.
2. Description of the Related Art
FIG. 5 is a schematic diagram showing a voltage detecting device of a conventional three-phase-batch type gas insulated switching device as disclosed, for example, in Japanese Patent Application Laid-open No. Hei 2-115770.
In FIG. 5, three-phase high-voltage conductors 3a through 3c are arranged in a regular triangle in an annular grounding container 1 filled with an insulation arc-extinguishing gas, such as SF6 gas. Provided between openings of the annular grounding container 1 connected to each other is a disc-like partition mounting plate 17 consisting of a metal material, and bolts (not shown) are inserted into the peripheral portions of the high-voltage conductors 3a through 3c for fastening and fixation.
This partition mounting plate 17 has coaxial through-holes 18a through 18c having a diameter which sufficiently allows insulation if the high-voltage conductors 3a through 3c are inserted, and single-phase spacers 19a through 19c made of insulating materials are respectively mounted to the through-holes 18a through 18c, supporting the high-voltage conductors 3a through 3c. 
Further, cylindrical grounding shields 16a through 16c situated coaxially with the through-holes 18a through 18c of the partition mounting plate 17 and mitigating the electric field of the mounting flange portions of the single-phase spacers 19a through 19c are fixed to the partition mounting plate 17 by fastening screws common to the single-phase spacers 19a through 19c. 
Further, cylindrical floating electrodes 15a through 15c are provided in the outer periphery of the high-voltage conductors 3a through 3c and inside the grounding shields 16a through 16c so as to be coaxial therewith, the floating electrodes 15a through 15c being secured to the respective grounding shields 16a through 16c through the intermediation of insulators 20.
The respective floating electrodes 15a through 15c are connected to transformer portions 24a through 24c arranged outside the annular grounding container 1 and containing auxiliary capacitors through the intermediation of conductors 23 extending through insulation sealing terminals 21 on the annular grounding container 1 respectively mounted at the closest position and through hole portions 22a through 22c provided in the grounding shields 16a through 16c. 
Further, the floating electrodes 15a through 15c are arranged at the centers of the grounding shields 16a through 16c with respect to the axial direction thereof, and the grounding shield length 1x is substantially not less than twice the length 1z of the floating electrodes 15a through 15c. 
In the above-described capacitor type voltage divider in the conventional gas insulated switching device, in order to restrain the influence from other phase, the grounding shields 16a through 16c are provided coaxially with the floating electrodes 15a through 15c, so that it is difficult to reduce the inter-phase distance of the high-voltage conductors 3a through 3c, thus making it difficult to achieve a reduction in the size of the three-phase-batch type gas insulated switching device. Further, the provision of the grounding shields 16a through 16c leads to an increase in the cost of the three-phase-batch type gas insulated switching device.
This invention has been made with a view to solving the above problems in the prior art. It is an object of this invention to provide a high-accuracy transformer for a gas insulated electrical apparatus which needs no grounding field and which has a simple structure.
According to the present invention, there is provided a transformer for a gas insulated electrical apparatus applicable to a gas insulated electrical apparatus in which a high-voltage conductor is arranged in an annular grounding container in which an insulation arc-extinguishing gas is sealed. The transformer comprises a capacitor type voltage divider having a floating electrode at a position opposed to a high-voltage conductor through an insulation sealing terminal mounted to a flange of a branch pipe provided on an inner surface of the annular grounding container. Also comprises an ohm resistor for voltage division which is mounted on an outer peripheral surface of the annular grounding container, one end of which is grounded and the other end of which is connected to the floating electrode through the capacitor type voltage divider, and which has an impedance which is sufficiently smaller than the impedance between the floating electrode and the annular grounding container. Further the transformer a converting portion which is connected in parallel to the ohm resistor for voltage division, which has an input impedance sufficiently larger than the resistance value of the ohm resistor for voltage division, and which converts the detection voltage of the high-voltage conductor to a voltage value that allows processing on the secondary side.
Therefore, it is possible to obtain a stable and highly accurate voltage divider free from the influence of assembly errors, etc., so that it is possible to reduce the inter-phase distance of the high-voltage conductor without adopting a structure in which a grounding shield is provided. Thus, it is possible to achieve a reduction in the size of the gas insulated switching device, and there is no need to provide a grounding shield, thereby achieving a reduction in cost.